How is this possible?

Impossible to say without knowing exactly what this data structure looks like.

It looks like this:

Here you have direct output from the service which this data come from, in json format

"annuitySettings":{"paymentMethodTypeCode":"","paymentMethodTypeDesc":"","paymentMethods":[]},

The problem here is the edge cases around empty lists and variants. The following snippet replicates that.

length(cast(type!Variant, {}))

Try to cast that empty list of dictionaries into a list of Maps. Not great, but works for me.

length(cast(a!listType(type!Map), local!data.annuitySettings.paymentMethods))

Thank you Stefan, it works.

I had solved the error with this

and(
        length(local!taller.resultado.contractSetting.annuitySettings.paymentMethods) = 1,
        a!isNotNullOrEmpty(local!taller.resultado.contractSetting.annuitySettings.paymentMethods)
      ),

Both solution are quite ugly 

I think that Appian engineers should work on this, it is not coherent that length funntion returns 1 on an empty array

Or you might utilize Stefan's rule 'isVoid' checking for empty structures (all sorts of). ri!value is of type 'Any Type'

if(
  a!isNullOrEmpty(ri!value),
  true,
  a!localVariables(
    local!typeNum: a!refreshVariable(
      value: runtimetypeof(ri!value),
      refreshAlways: true
    ),

    /* Strings */
    if(
      local!typeNum = 'type!{http://www.appian.com/ae/types/2009}Text',
      /* Ignore white space */
      len(trim(ri!value)) = 0,

      /* List of Variant */
      if(
        local!typeNum = 'type!{http://www.appian.com/ae/types/2009}Variant?list',
        all(a!isNullOrEmpty(_), a!flatten(ri!value)),

        /* Lists */
        if(
          /* Try to take first item in list and cast it to a list of itself */
          /* then compare its datatype to the incoming data type */
          local!typeNum = runtimetypeof(cast(runtimetypeof({index(ri!value, 1, null)}), ri!value)),

          /* Flatten takes care of lists in lists in lists in ... */
          /* List has zero non-null items, length() already ignores null values*/
          length(a!flatten(ri!value)) = 0,

          /* Map */
          if(
            local!typeNum = 'type!{http://www.appian.com/ae/types/2009}Map',
            /* Has no keys at all or all values are null */
            or(
              length(a!keys(ri!value)) = 0,
              all(a!isNullOrEmpty(_), index(ri!value, a!keys(ri!value), null))
            ),

            /* DataSubsets or Maps returned by queryRecordType */
            if(
              or(
                local!typeNum = 'type!{http://www.appian.com/ae/types/2009}DataSubset',
                and(
                  local!typeNum = 'type!{http://www.appian.com/ae/types/2009}Map',
                  contains(a!keys(ri!value), "data"),
                )
              ),
              a!isNullOrEmpty(ri!value.data),

              /* No idea what this could be so it is considered to not be null */
              false
            )
          )
        )
      )
    )
  )
)

Or you might utilize Stefan's rule 'isVoid' checking for empty structures (all sorts of). ri!value is of type 'Any Type'

if(
  a!isNullOrEmpty(ri!value),
  true,
  a!localVariables(
    local!typeNum: a!refreshVariable(
      value: runtimetypeof(ri!value),
      refreshAlways: true
    ),

    /* Strings */
    if(
      local!typeNum = 'type!{http://www.appian.com/ae/types/2009}Text',
      /* Ignore white space */
      len(trim(ri!value)) = 0,

      /* List of Variant */
      if(
        local!typeNum = 'type!{http://www.appian.com/ae/types/2009}Variant?list',
        all(a!isNullOrEmpty(_), a!flatten(ri!value)),

        /* Lists */
        if(
          /* Try to take first item in list and cast it to a list of itself */
          /* then compare its datatype to the incoming data type */
          local!typeNum = runtimetypeof(cast(runtimetypeof({index(ri!value, 1, null)}), ri!value)),

          /* Flatten takes care of lists in lists in lists in ... */
          /* List has zero non-null items, length() already ignores null values*/
          length(a!flatten(ri!value)) = 0,

          /* Map */
          if(
            local!typeNum = 'type!{http://www.appian.com/ae/types/2009}Map',
            /* Has no keys at all or all values are null */
            or(
              length(a!keys(ri!value)) = 0,
              all(a!isNullOrEmpty(_), index(ri!value, a!keys(ri!value), null))
            ),

            /* DataSubsets or Maps returned by queryRecordType */
            if(
              or(
                local!typeNum = 'type!{http://www.appian.com/ae/types/2009}DataSubset',
                and(
                  local!typeNum = 'type!{http://www.appian.com/ae/types/2009}Map',
                  contains(a!keys(ri!value), "data"),
                )
              ),
              a!isNullOrEmpty(ri!value.data),

              /* No idea what this could be so it is considered to not be null */
              false
            )
          )
        )
      )
    )
  )
)